This invention relates to an improved hybrid rocket fuel, and is particularly directed to a hybrid rocket fuel containing a high percentage of hexamethylenetetramine, and to its method of production.
Hybrid fuel systems for rocket propulsion are those in which 1) the fuel and oxidizer are stored separately, and 2) either the fuel or the oxidizer is a solid, while the other component is a liquid. Hybrids appear to have advantages over both solid and liquid propulsion systems in cost, safety, reliability and in producing environmentally benign combustion products. However, a major limitation in development of hybrid rocket propulsion has been the solid fuel regression rate. The regression rate is the rate at which the surface erodes, that is, the rate at which molecules at the surface (often high molecular weight molecules) are decomposed into smaller fragments and/or vaporized, so they can subsequently be burned. It is desirable to obtain as high a regression rate as possible.
U.S. Pat. No. 4,206,006 to Ratz teaches that hexamethylenetetramine as well as nitrited products thereof, can be used as an ignition catalyst in hybrid rocket fuels to increase the reaction speed with respect to the oxidation medium. The patent discloses that hexamethylenetetramine can be used as a catalyst in a proportion of about 5% to 50% based on the total mass of the fuel "to vary within wide limits the speed of reaction with the oxidizer". The hexamethylenetetramine in Ratz is added to the fuel which is in the form of a liquid prepolymer of butadiene with terminal carboxyl groups and a trifunctional cross-linking agent such as phosphoric acid tripropyleneimide.
It is an object of the invention to provide an improved hybrid fuel for rocket propulsion.
Another object is the provision of a hybrid rocket fuel containing hexamethylenetetramine and an elastomeric binder system, e.g. polybutadiene, polyether, or polyester, crosslinked via polyurethane linkages, wherein the hexamethylenetetramine functions as the major fuel component.
Yet another object is to provide an improved hybrid rocket fuel of the above type employing a superior elastomeric binder system, differing both in prepolymer and cross-linking agent from that of the above Ratz patent, and exhibiting improved moisture resistance and superior aging characteristics compared to that of Ratz.
Still another object is the provision of a hybrid rocket fuel containing hexamethylenetetramine as the major fuel component and having improved regression rate and improved motor performance.
A further object is to provide a method for producing the improved hybrid fuel for rocket propulsion of the invention.
Other objects and advantages will appear hereinafter.